In simplest form, ink jet image printing devices which print a large number of small dots to create the desired image operate by moving a print head linearly back and forth across a print medium, for example, a sheet of paper, and printing dots with that print head according to an image data stream. The dots are printed in a densely packed pattern that, when viewed by human eyes, appears as the image represented by the image data stream. This image can be alpha-numeric text, a picture or graphic, or a combination of the two. Similarly, in a laser printing device, the laser is scanned linearly back and forth across a print drum to create areas or pixels (“dots”) to which toner will adhere when applied, thus creating the image specified by the image data stream.
In order to provide a high quality image, a continuum of shades between black and white, or between extreme shades of each color, must be available so as to be rendered by the printing device. These shades between the extremes are known as halftones or gray-scale levels. A half-tone may be specified by absorptance. “Absorptance” refers to the fraction of photons absorbed by a dot or portion of the printed image. The higher the fraction of absorbed photons, as distinguished from reflected photons, the darker that portion of the printed image appears. Consequently, within the image printed by the printing device there must be a method of making the printed dots generate gradations or halftones to create a high-quality image.
Half-toning can be accomplished by a number of methods that involve modulating the characteristics of the dots printed. There are two main dot characteristics that can be varied to effect the half-tone or gray-scale of a portion of an image printed. These characteristics are the size of the dots and the density of the dots. Both of these characteristics can be modulated simultaneously to control how light or dark, in a gray or color scale, a particular section of the printed image appears. With printers that are stable and have less dot gain, the advantage of dot size modulation is most significant in highlights where smaller dots produce less visible highlight textures. With printers that are less stable and/or have more dot gain, using larger dots in the mid-tones may also result in improved quality.
In many printing devices, a relatively large amount of data is required at all stages of data processing to communicate an image to be printed. Consequently, there is a present need in the art for a method and device that can, at least at some phase of the data processing, decrease the amount of data that must be stored and transmitted to represent the image being printed without sacrificing the quality of the image.